FIG. 1 shows an example of a multi-standard reception architecture. Firstly, an antenna 1 receives the signals. A switching stage 2, including several switches, directs the received signal to one of the appropriate pass-band filters 3a-3c to recover only the desired frequency signal at the output.
Switches used in the switching stage 2 may, for example, comprise transistors (for example, field effect transistors based on gallium arsenide), and PIN diodes. These switches have the advantage that they are easy to implement, but they may not be directly integrated. Accordingly, they may be transferred onto the circuit. Furthermore, they may introduce large power losses on the signal to be routed, particularly, at high frequencies, such as, frequencies used for wireless communications. These switches may comprise Micro Electro Mechanical Systems (MEMS). This type of switch has good isolation and contact properties, but the switching voltages used are usually high (>12V), and their production is complex. Furthermore, the size of these MEMS remains fairly large, for example, of the order of 500*500 μm2.
Typically, the filters used in this type of architecture, such as, filters 3a-3c in FIG. 1, are Bulk Acoustic Wave (BAW) filters. These BAW filters are made by coupling piezoelectric resonators. A piezoelectric resonator comprises a resonant layer comprising piezoelectric material arranged between two electrodes.
BAW filters may have electrical coupling. The electrodes of the piezoelectric resonators are coupled electrically between themselves. Thus, the signal to be filtered is propagated from one resonator to another passing through electrical connections coupling the resonator electrodes between themselves. Coupling can be done in series and/or in parallel to obtain Ladder filters, or in lattice to obtain Lattice filters. BAW filters may also have acoustic coupling. The signal to be filtered then propagates from one resonant layer to another resonant layer directly or through an acoustic propagation medium. It may use Stacked Crystal Filters (SCF) and Coupled Resonator Filters (CRF).
An example of an SCF 4 is shown in FIG. 2. This filter 4 includes a substrate 5, on which a Bragg mirror 6 is stacked with an output electrode 7, a first piezoelectric resonant layer 8, a central electrode 9, a second piezoelectric resonant layer 10, and an input electrode 11. A ground 14 is connected to the central electrode 9, common to the two piezoelectric layers 8, 10. The output electrode 7, the central electrode 9, and the first piezoelectric layer 8 form a first piezoelectric resonator. While the input electrode 11, the central electrode 9, and the second piezoelectric layer 10 form a second piezoelectric resonator acoustically coupled with the first piezoelectric resonator. The input signal is applied between the input electrode 11 and the ground 14. The signal then propagates through the two piezoelectric layers 10, 8. The thickness of each piezoelectric layer may, for example, be equal to about a half wavelength λ/2. The signal recovered between the output electrode 7 and the ground 14 is actually the signal with the wavelength λ.
The Bragg mirror 6 comprises a stack of alternating layers with high and low acoustic impedance preventing propagation of this signal in the substrate 5, particularly, by reflecting signals with wavelength equal to λ. This filter 4 is used to obtain a narrow passband at the output (passband about 50 MHz where f=1.5 GHz). These SCF filters are sometimes made without a Bragg mirror, for example, directly on a membrane. But in this case, the output spectrum contains harmonics with a parasite wavelength equal to λ/2 and 3λ/2. A filter similar to the one shown in FIG. 2 is disclosed in U.S. Pat. No. 5,821,833 to Lakin. The addition of the Bragg mirror 6 eliminates these parasite harmonics in the output spectrum that are then dissipated in the substrate 5.
But the major disadvantage of these filters is that they may not be used for impedance matching, for example, when an impedance of 50Ω is used at the input, and an impedance of 200Ω is used at the output. Additional passive components, such as, inductors and capacitors then have to be used, for which the quality factor is important and thus creating additional constraints, for example, cost and size. Furthermore, with this type of resonator, it may be impossible to carry out a “single” type line conversion, in other words, non-differential to a differential type line. Once again, additional passive components may have to be used with exactly the same constraints as those mentioned above.
An example CRF 15 according to prior art is shown in FIG. 3. The CRF 15 includes a substrate 5, a Bragg mirror 6, and two piezoelectric layers 8, 10, identical to those shown in FIG. 2. In this CRF 15, each of the piezoelectric layers 8, 10 is arranged between two electrodes, 17a, 17b and 18a, 18b respectively. Thus, the piezoelectric layer 8 and the electrodes 17a and 17b form a first piezoelectric resonator, and the piezoelectric layer 10 and the electrodes 18a and 18b form a second piezoelectric resonator. Several acoustic coupling layers 16 are arranged between the two electrodes 17a, 18b themselves arranged between the two piezoelectric layers 8, 10, thus coupling the two piezoelectric resonators.
This CRF 15 operates in a differential mode. The input signal is applied between the two electrodes 18a, 18b in the second piezoelectric layer 10 and passes through the acoustic coupling layers 16. The output signal is recovered differentially between the two electrodes 17a, 17b in the first piezoelectric layer 8. The CRF 15 can create a wider passband at the output than the SCF 4, and the input can be electrically decoupled from the output. By modifying the number of acoustic coupling layers 16 and the nature of these layers, it is possible to modify the acoustic coupling so as to optimize the passband recovered at the output. With this type of filter, it is also possible to achieve impedance matching, for example, to change from 50Ω at the input to 200Ω at the output, as described in document “Coupled Bulk Acoustic Wave Resonator Filters: Key Technology for single-to-balanced RF filters” by Fattinger et al., IEEE MTT-S Digest 2004, pages 927 to 929. However, the passband obtained with this type of filter is wider than the passband obtained with an SCF (Passband about 70 MHz where f=1.5 GHz). Such a filter is disclosed in U.S. Pat. No. 6,720,844 to Lakin.
FIG. 4 shows another example filter 19. This filter 19 is made by putting two CRFs 15a, 15b into series, for example, identical to the CRF 15 in FIG. 3. Two piezoelectric resonators 26a, 26b, belonging to the first CRF 15a and the second CRF 15b respectively, each including a piezoelectric layer 20a, 20b, respectively, are arranged on the acoustic coupling layers 16 common to the two CRFs 15a, 15b. The first resonator 26a includes two electrodes 21a, 21b through which an input signal is introduced. The second resonator 26b includes two electrodes 22a, 22b through which the output signal is recovered from the filter 19. The electrodes 21a, 21b, 22a, 22b of the two resonators are not connected together.
A third and a fourth piezoelectric resonator 27a, 27b belong to the first CRF 15a and the second CRF 15b, respectively. Each piezoelectric resonator includes a piezoelectric layer 23a, 23b, respectively, for which the electrodes 24a, 24b and 25a, 25b are connected to each other in pairs. Thus, the signal propagates in the first resonator 26a, then in the acoustic coupling layers 16, the third resonator 27a, the fourth resonator 27b, and then leaves through the second resonator 26b. This filter 19 may give a better selectivity for filtering than the CRF 15 in FIG. 3.